The use of gas turbines for the generation of electrical energy with optional cogeneration of steam is commercially desirable. Such gas turbines normally involve at least one compression unit, at least one combustion unit and at least one expansion unit or turbine. Efficiencies of various systems vary dependent upon, among other factors, the heat value of the gas combusted.
There is an increasing interest in the use of gases of lower than conventional heat values, or low BTU gases, in gas turbine generator systems. Such low BTU gas, for example, can be produced by air blown gasification of low-grade fuels, such as peat, and/or combustible wastes, which contain significant quantities of oxygen and water moisture and which may be cooled by the evaporation of water spray from the gasification temperature to a temperature suitable for use in the combustion unit of a gas turbine system.
When such low heat value gases are used in a gas turbine system, it is usually necessary to bleed a portion of the air passing through the compression unit of the system. The large mass of moist, low BTU gas fuel required in the combustion unit of the system to reach the design expander unit inlet temperature results in an increased mass flow through the expansion unit and an increased expansion unit inlet pressure. Air bleed from the compression unit outlet may then be required to prevent a surge in the compression unit by limiting the expansion unit flow and thus the pressure at the outlet of the compression unit and inlet of the expansion unit. Air bleed of up to about 20 percent may be required, depending upon the BTU per cubic feet content and temperature of the fuel gas, to prevent surge. This air bleed represents an energy loss, in that the air may have been compressed from atmospheric pressure to 8-20 atmospheres by the compression unit. Typically, the energy loss is up to 30 percent of that produced by the gas turbine engine. Alternatively, the air can be expanded in an auxiliary device to regain the energy of the compression unit, but such auxiliary expansion requires additional equipment and expense.
Mangan et al., in U.S. Pat. No. 3,150,487 disclose a process for operating a gas turbine system containing a compression unit, a combustion unit and an expansion unit wherein the discharge gases are used to generate steam to operate a steam turbine and, directly or indirectly, to heat the air supplied to the compression unit. Mangan et al. allege improvements in efficiency of an integrated power plant employing a gas turbine with a steam turbine driven by steam generated by exhaust gas from the gas tubine.
La Haye in U.S. Pat. No. 3,422,800, relate to an improved control system for a gas turbine and a waste heat boiler system similar to that of Mangan et al. which independently controls steam generating capabilities of the boiler despite variations in gas turbine load.
Rice's invention in U.S. Pat. No. 3,703,807 is an improvement in the process of Mangan et al. in that part of the boiler stack gas is mixed with ambient air entering the gas section prior to filtration, resulting in a reduction of the loss of heat through the stack, thereby increasing the efficiency of the power plant.
Collet in U.S. Pat. No. 4,426,842 also heats air supplied to a compression unit wherein the invention relates to a system for heat recovery in which part of the waste heat in the combustion gases after their expansion is returned with recovery means into the combustion air flow.
Mangan et al., La Haye and Rice do not refer to any specific fuels used in their combustion units. Collet uses a fuel-like natural gas or fluid fuel in his combustion unit.